This invention relates to novel gel compositions which are capable of discontinuous volume change of several hundred times induced by infinitesimal changes in environment.
Gel is a form of material between the liquid and solid state. It consists of a crosslinked network of long polymer molecules with liquid molecules trapped within the network. Gels play important roles in various aspects of our everyday life.
In chemistry and biochemistry, gels are used extensively as matrices for chromatography and electrophoresesanalytical methods that separate molecules according to their molecular weights and charges. In these techniques, the pore size of the crosslinked polymer network plays an essential role in its sieving effects.
Gels also are important intermediate products in polymer products such as rubbers, plastics, glues and membranes.
In 1973, a new technique of light scattering spectroscopy was first introduced to gel studies. It was demonstrated that by measuring the intensity and the time dependence of fluctuations of laser light scattered from a gel, it is possible to determine the viscoelastic properties of the gel, that is, the elasticity of the polymer network and the viscous interaction between the network and the gel fluid. Recently, with the help of this powerful technique, very interesting phenomena in permanently crosslinked gels have been found: as the temperature is lowered, the polymer network becomes increasingly compressible, and at a certain temperature, it becomes infinitely compressible. At the same time, the effective pore size of the network increases and diverges. It is also observed that the volume of polyacrylamide gels ranges reversibly by a factor as large as several hundred by an infinitesimal change in external conditions such as solvent composition or temperature. Tanaka, Physical Review Letters, Vol. 40, No. 12, pp. 820-823, 1978 and Tanaka et al, Physical Review Letters, Vol. 38, No. 14, pp 771-774, 1977; Tanaka et al, Physical Review Letters 5, Vol. 45, pg 1636, 1980; Ilovsky, Macromolecules, Vol. 15, pg 782, 1982 and Hrouz et al, Europ. Polym. J., Vol. 17, pg 361, 1981.
It is known that copolymers of acrylamide and sodium acrylate in gel form are capable of drastic volume and it is also known that the gel showed continuous volume change when the concentration of the acrylic acid component, which was an ionizable group, was smaller than the critical value, while the change became discontinuous when the concentration of the acrylic acid component was greater than the critical value.
According to the conventional knowledge as described above, in order to prepare a polymer gel having the phase transition function with discontinuous volume change, it was believed necessary that the polymer contained a considerably large amount of the ionizable group.
The volume change of the conventional gels having an ionic group described above significantly depends on hydrogen ion concentration (pH) of the liquid medium to be used. Therefore, the conventional ionic gel could not be used when pH of the liquid medium could not be controlled sufficiently.
It is also known that polymers containing no ionizable group formed from a monomer containing a predominant amount of a N-substituted (metha)acrylamide and a crosslinking agent exhibit a drastic volume in water or mixtures of a solvent and water in response to change in liquid solvent composition. These gels do not depend upon hydrogen ion concentration of the liquid solvent. While these gels can be useful as a switching device or artifical muscle due to their ability to undergo discrete volume change cause by minute environmental changes, it would be desirable to provide gels which undergo greater volume changes in order to maximize the efficiency of the functions.